TURBINES
Turbines Turbines are defined as the hydraulic hydraulic machines machines which convert hydraulic hydraulic energy into mechanical mechanical energy. energy. This mechanical mechanical energy is used in running an electric electric generator which is directly coupled to the shaft of the turbine. Thus the mechanical energy is converted into electrical energy. PUMPS
Pumps are defined as the hydraulic machines which convert mechanical energy into hydraulic energy are called pumps. HYDRO-ELECTRIC POWER PLANT
Hydraulic turbines are the machines which use the energy of water and convert it to mechanical energy. energy. The mechanical mechanical energy developed by a turbine turbine is used in running an electric generator which is directly coupled to the shaft of the turbine. The electric generator thus develops electric power, which is known as hydro-electric power. General Layout of a Hyraul!" Po#er Plant
Fig. shows a general layout of hydraulic power plant, which consists of:
!i" !i"
# dam dam cons constr truc ucte tedd acr acros osss a rive riverr to to stor storee wat water er
!ii" !ii"
Pipe Pipess of of lar large ge diam diamet eter erss cal calle ledd pen penst stoc ocks ks
!iii" !iii"
Turbi Tu rbines nes having having differ different ent types types vanes vanes fitted fitted to the wheel wheel
!iv" !iv"
Tail Tail race race , which which is a channe channell which which carries carries water water away away from the water water
$%F&'&T&(') (F H%#$) . Gro$$ Hea: The difference between the head race level and tail race level when no water is flowing is known as *ross head. &t is denoted by Hg +. Net Hea : &t is also called effective head and is defined as the head available at the inlet of the turbine. &t is denoted by H 'et Head H Hg hf W%ere
Hg *ross head hf Head loss due to friction friction . f .L.V + h f = d × + g
CLASSI&ICATION CLASSI&ICATION O& HYDRAULIC TURBINES
The hydraulic turbines classified are . #ccordi #ccording ng to the the type type of energ energyy at inle inlett a" &mpu &mpuls lsee turb turbin inee and and b" /eaction turbine +. #ccordi #ccording ng to the the directi direction on of flow flow through through runner runner a" Tangent ngentia iall flow flow turbi turbine ne b" /adial flow turbine c" #0ia #0iall flo flow w turb turbin inee and and d" 1i 1i0e 0edd flow flow turbi turbine ne 2. #ccordi #ccording ng to the the head head at the the inlet inlet of of turbin turbinee a" High High head head turb turbin inee b" 1edium head turbine and c" 3o 3ow w head head turb turbin inee . #ccordi #ccording ng to to the the specifi specificc speed speed of the the turbi turbine ne +
a" 3ow specific speed turbine b" 1edium specific speed turbine and c" High specific speed turbine IMPULSE TURBINE '
&f at the inlet of the turbine, the energy available is only kinetic energy, the turbine is known impulse turbine. E(a)*le'
Pelton wheel turbine REACTION TURBINE'
&f at the inlet of the turbine, the water possesses kinetic energy as well as pressure energy, the turbine is known as reaction turbine. E(a)*le'
Francis turbine,
4aplan turbine.
RADIAL &LOW TURBINE '
&f the water flow in the radial direction through the runner, the turbine is called radial flow turbine /adial flow turbine
&nward radial flow turbine
(utward radial flow turbine.
INWARD RADIAL &LOW TURBINE '
&f the water flows from outward to inward, radially the turbine is known as inward radial flow turbine. OUTWARD RADIAL &LOW TURBINE '
&f the water flow radially from inward to outwards, the turbine is known as outward radial flow turbine. 2
A+IAL &LOW TURBINE '
&f the water flow through the runner along the direction parallel to the a0is of rotation of the runner, the turbine is called a0ial flow turbine. MI+ED &LOW TURBINE'
&f the water flows through the runner in the radial direction but leaves in the direction parallel to a0is of rotation of the runner, the turbine is called mi0ed flow turbine. TANGENTIAL &LOW TURBINE '
&f the water flows along the tangent of the runner, the turbine is known as tangential flow turbine.
E&&ICIENCIES O& A TURBINE
The following are the important efficiencies of a turbine . Hydraulic efficiency !η h" +. 1echanical efficiency !η m" 2. 5olumetric efficiency !η v" . (verall efficiency !η o" HYDRAULIC E&&ICIENCY, η %
&f is defined as the ratio of power given by water to the runner (f a turbine to the power supplied by the water at the inlet of the turbine. η h
=
Power deliverd to runner Power supplied at inlet
MECHANICAL E&&ICIENCY !η m"
&f is defined the ratio of the power available at the shaft of the turbine to the power delivered to the runner. η m
Power at the shaft of the turbine Power delivered by water to the runner
.OLUMETRIC E&&ICIENCY , η /
&f is defined the ratio of the volume of the water actually striking the runner to the volume of water supplied to the turbine.
η v
5olume of water actually striking the water 5olume of water supplied to the turbine
O.ERALL E&&ICIENCY , η o"
&f is defined as the ratio of power available at the shaft of the turbine to the power supplied by the water at the inlet of the turbine. η o
5olume available at theshaft of the turbine Power supplied at the inlet of the turbine
6
PELTON WHEEL
The only hydraulic turbine of the impulse type in common use, is named after an #merican engineer 3aster # Pelton, who contributed much to its development around the year 778. Therefore this machine is known as Pelton turbine or Pelton wheel. &t is an efficient machine particularly suited to high heads. Pelton wheel is well suited for operating under high heads. # pelton turbine has one or more no99les discharging ets of water which strike a series of buckets mounted on the periphery of a circular disc. The runner consists of a circular disc with a number of buckets evenly spaced round its periphery. The buckets have a shape of a double semiellipsoidal cups. The pelton bucket is designed to deflect the et back through ;6° which is the ma0imum angle possible without the return et interfering with the ne0t bucket The rotor consists of a large circular disc or wheel on which a number !seldom less than 6" of spoon shaped buckets are spaced uniformly round is periphery as shown in Figure . The wheel is driven by ets of water being discharged at atmospheric pressure from pressure no99les. The no99les are mounted so that each directs a et along a tangent to the circle through the centres of the buckets . $own the centre of each bucket, there is a splitter ridge which divides the et into two e
;
The main parts of the pelton turbine are= . 'o99le and flow regulating arrangement +. /unner and buckets 2. >asing and . ?reaking et No00le an flo# Re1ulat!n1 Arran1e)ent
The amount of water striking the buckets !vanes" of the runner is controlled by providing a spear in the no99le as shown in fig. The spear is a conical needle which is operated either by a hand wheel or automatically in an a0ial direction depending upon the si9e of the unit. @hen the spear is pushed forward into the no99le the amount of water striking the runner is reduced. (n the other hand, if the spear is pushed back, the amount of water striking the runner increases.
A
Runner #!t% Bu"2et$
Fig shows the runner of a pelton wheel. &t consists of a circular disc on the periphery of which a number of buckets evenly spaced are fi0ed. The shape of the buckets is of a double hemispherical cup or bowl. %ach bucket is divided into two symmetrical parts by a dividing wall which is known as splitter.
The et of water strikes on the splitter. The splitter divides the et into two e
7
The function of the casing is to prevent the splashing of the water and to discharge water to tail race. &t also acts as safeguard against accidents. &t is made of cast iron or fabricated steel plates. The casing of the pelton wheel does not perform any hydraulic function. Brea2!n1 3et
@hen the no99le is completely closed by moving the spear in the forward direction the amount of water striking the runner reduces to 9ero. ?ut the runner due to inertia goes on revolving for a long time. To stop the runner in a short time, a small no99le is provided which directs the et of water on the back of the vanes. This et of water is called breaking et. L!)!tat!on of a Pelton Tur4!ne'
." The Pelton wheel is efficient and reliable when operating under large heads. +." To generate a given output power under a smaller head, the rate of flow through the turbine has to be higher which re
Fig shows the shape of the raues or buckets of the pelton wheel. The set of water from the no99le strikes the bucket at the splitten which splits up the set into two parts. These part of the set, glides over the inner surfaces and comes out at the outer edge.
B
The inlet velocity triangle is drawn at the splitter and outlet velocity triangle is drawn at the outer edge of the bucket.
3et H 'et Head artily on the pelton wheel. Hg hf W%ere'
Hg *ross head FLV + hf D × + g $ $iameter of the wheel ' )peed of the wheel 5 5elocity of et at inlet 5 + gH u u u+
π DN
;8
The velocity triangle at inlet will be a straight line where 5r 5 u 5 C 5w 5 α
=
8 and θ = 8
OUTLET .ELOCITY TRIANGLE
5r + 5r 8
5w+ 5r + >os φ − u+ The force e0erted by the et of water. F0 ρ aV [ Vw + Vw+ ] @ork done by the et on the runner per second F0 D u ρ aV [ Vw + Vw+ ] × u 'mEsec Power given to the runner by the et
ρ aV [ Vw + Vw+ ] × u
888
× kw
@ork done Es per unit weight of water striking E s
=
ρ aV [ Vw + Vw+ ] × u
@eight of water striking E s ρ aV [ Vw + Vw+ ] ρ aV × g
= [ Vw + Vw+ ] × u g
Hyraul!" eff!"!en"y'
η h K .E
=
@ork done per second K .Eofjetper sec ond
of et per second η h
=
! ρ aV " × V + +
ρ aV [ Vw + Vw+ ] × u
ρ aV × V + + + [ Vw + Vw+ ] V +
×u
'ow 5w 5 5r 5 u 5r + 5 u 5w+ 5r + >os φ − u +
!5 u " cos φ − u )ubstituting the values of 5w and 5w+ η h
=
+ [ V + !V − u " cos φ − u ] × u V +
= η h
=
+ [ V − u + !V − u " cos φ ] V +
×u
+!V − u " + cos φ Gu V +
Po!nt$ to 4e Re)e)4ere for *elton #%eel
i"
The velocity of the et at inlet 5 >v+gH >v >o-efficiency of velocity 8.B7 or 8.BB H 'et head on turbine φ + gH φ π DN
ii" The velocity of wheel !u" is given by u ;8 D m
φ + gH
d
Z 6 I
D
+d
6 I8.6m
where φ )peed ratio The value of speed ratio varies from 8.2 to 8.B.
iii"The angle of deflection of the et through buckets is taken at ;6 8 if no angle of deflection is given iv" The mean diameter or the pitch diameter $ of the pelton wheel is given by u
π DN
;8
v" Jet ratio it is defined as the ratio of the pitch diameter $ of the pelton wheel to the diameter of the et d. &t is denoted by KmL and is given by
+
m
D d
vi" 'umber of buckets on a runner is given by Z 6 I
D
+d
6 I8.6m
@here m et ratio vii" 'umber of ets. &t is obtained by dividing the total rate of flow through the turbine by the rate of flow of water through a single et. DESIGN O& PELTON WHEEL
$esign of pelton wheel means the following data is to be determine. . $iameter of the et !d" +. $iameter of wheel !$" 2. @idth of the buckets which is 60d . $epth of the buckets which is .+0d 6. 'umber of buckets on the wheel )i9e of buckets means the width and depth of the buckets. RADIAL &LOW REACTION TURBINES
/eaction turbine means that the water at the inlet of the turbine possesses kinetic energy as well as pressure energy. #s the water flows through the runner, a part of pressure energy goes on changing into kinetic energy. Thus the water through the runner is under pressure. The runner is completely enclosed in an air tight casing and casing and the runner is always full of water. /adial flow turbine are those turbines in which the water flows in the radial direction. The water may flow radially from outwards to inwards or from inwards to outwards Ma!n *art$ of ra!al flo# Rea"t!on Tur4!ne
The main parts of a radial flow reaction turbine are: . >asing +. *uide mechanism 2. /unner, and 2
. $raft tube.
Ca$!n1'
>asing and runner are always a full of water. The water from the penstocks enters the casing which is of spiral shape in which area of cross-section of the casing goes on decreasing gradually. The casing completely surrounds the runner of the turbine. The casing is made of spiral shape , so that the water enter the runner at constant velocity through out the circumference of the runner. The casing is made of concrete, cast steel or plate steel. Gu!e Me"%an!$)'
&t consists of a stationary circular wheel all round the runner of the turbine. The stationary guide vanes are fi0es on the guide mechanism. The guide vanes allow the water to strike the vanes fi0ed on the runner without shock at inlet. #lso by a suitable arrangement, the width between two adacent vanes of guide mechanism can be altered so that the amount of water striking the runner can be varied. Runner'
&t is a circular wheel on which a series of radial curved vanes are fi0ed. The surface of the vanes are made very smooth. The radial curved are so shaped that the
water enters and leaves the runner without shock. The runners are made of cast steel, cast iron or stainless steel. They are keyed to the shaft. Draft-tu4e'
The pressure at the e0it of the runner of a reaction turbine is generally less than atmospheric pressure. The water at e0it cannot be directly discharged to the tail race. # tube or pipe of gradually increasing area is used for discharged water from the e0it of the turbine to the tail race. This tube of increasing area is called draft tube. I)*ortant Def!n!t!on$
The following terms are generally used in case of reaction radial flow turbines which are defined as: S*ee Rat!o'
u
The speed ratio is defined as
+ gH
@here u Tangential velocity of wheel at inlet.
&lo# rat!o'
The ratio of the velocity of flow at inlet !5f" to the velocity given + gH is known as flow ratio or it is given as
V f
+ gH
@here H head on turbine
D!$"%ar1e of t%e Tur4!ne7
The discharge through a reaction radial flow turbine is given by M π $?5f π $+?+5f+ @here $ $iameter of runner at inlet ? @idth of runner at inlet 5f 5elocity of flow at inlet 6
$+,?+,5f+ >orresponding values at outlet 'ote i"
&f the thickness of vanes are taken in to consideration, then the area through which flow takes places is given by ! π $ nt" @here n 'umber of vanes on runner t Thickness of each vane
Then discharge M ! π $ nt"?5f ii" The head !H" on the turbine is given by H
p
ρ g
I
V +
+ g
INWARD RADIAL &LOW TURBINE
&f the water flows from outwards to inwards through the runner, the turbine is known as inward radial flow turbine. The guiding wheel consists of guide vanes which direct the water to enter the runner which consists of moving vanes.
The water flows over the moving vanes in the inward radial direction and is discharged at the runner diameter of the runner. The outer diameter of the runner is the inlet and the inner diameter is the outlet. .elo"!ty Tr!an1le$ an Wor2 one 4y #ater on runner Let
;
. w 5elocity of whirl at
5w+ 5elocity of whirl at outlet u,u+ tangential velocity of wheel at inlet and outlet θ /unner vane angle at inlet
5 #bsolute velocity of water leaving the guide vanes 5r /elative velocity of water entering the runner blade φ 5ane angle at outlet
5f and 5f+ velocity of flow at inlet and o u
π D N
,
;8
u +
π D N +
;8
@here $ outer diameter of runner $+ inner diameter of runner ' )peed of the turbine in r.p.m.
The work done per second per unit weight of water per second
@ork done per second @eight of water striking per second ρ Q [ V u w
±V
w+
u+ ]
ρ Qg
A
@
[V u ±V u ] w
w+
+
g
'ote i"
&f β is an obtuse angle then ve sign is taken
ii"
&f β B88, then V 8 and work done per second unit weight strikingEs becomes w+
as @
[V u ] w
g
iii"
&f the discharge is radial at outlet , then V 8
iv"
Hydraulic efficiency
w+
η
[V u ]
h
w
gH
Out#ar ra!al &lo# rea"t!on Tur4!ne
The outward radial flow reaction turbine in which the water from casing enters the stationary guide wheel. The guide wheel consists of guide vanes which direct water to enter the runner which is around the stationary guide wheel. The water flows through the vanes of the runner in the outward radial direction and is discharge at the outer diameter of the runner. The inner diameter of the runner is inlet and outer diameter is the outlet.
The velocity triangles at inlet and outlet will be drawn by the same procedure as inward flow turbine. &RANCIS TURBINE
7
The inward flow reaction turbine having radial discharge at outlet is known as Francis Turbine, after the name of J.?. Francis an #merican engineer who in the beginning designed inward radial radial flow reaction type of turbine. &n the Francis turbine, the water enters the runner of the turbines in the radial direction at outlet and leaves in the a0ial direction at the inlet of the runner. # Francis turbine comprises mainly the four components: !i" sprical casing, !ii" guide on stay vanes, !iii" runner blades, !v"
draft-tube
S*!ral Ca$!n1 '
1ost of these machines have vertical shafts although some smaller machines of this type have hori9ontal shaft. The fluid enters from the penstock !pipeline leading to the turbine from the reservoir at high altitude" to a spiral casing which completely surrounds the runner. This casing is known as scroll casing or volute. The cross-sectional area of this casing decreases uniformly along the circumference to keep the fluid velocity constant in magnitude along its path towards the guide vane.
B
This is so because the rate of flow along the fluid path in the volute decreases due to continuous entry of the fluid to the runner through the openings of the guide vanes or stay vanes. Gu!e or Stay /ane'
The basic purpose of the guide vanes or stay vanes is to convert a part of pressure energy of the fluid at its entrance to the kinetic energy and then to direct the fluid on to the runner blades at the angle appropriate to the design. 1oreover, the guide vanes are pivoted and can be turned by a suitable governing mechanism to regulate the flow while the load changes. The guide vanes are also known as wicket gates. The guide vanes impart a tangential velocity and hence an angular momentum to the water before its entry to the runner. The flow in the runner of a Francis turbine is not purely radial but a combination of radial and tangential. The flow is inward, i.e. from the periphery towards the centre. The height of the runner depends upon the specific speed. The height increases with the increase in the specific speed. The main direction of flow change as water passes through the runner and is finally turned into the a0ial direction while entering the draft tube. Draft tu4e'
The draft tube is a conduit which connects the runner e0it to the tail race where the water is being finally discharged from the turbine. The primary function of the draft tube is to reduce the velocity of the discharged water to minimi9e the loss of kinetic energy at the outlet. This permits the turbine to be set above the tail water without any appreciable drop of available head. # clear understanding of the function of the draft tube in any reaction turbine, in fact, is very important for the purpose of its design. The purpose of providing a draft tube will be better understood if we carefully study the net available head across a reaction turbine.
+8
The velocity triangle at inlet and outlet of the Francis turbines are drawn in the same way as incase of inward flow reaction turbine. #s in case of Francis turbine, the discharge is radial at outlet, the velocity of whirl at outlet5 w+ will be 9ero.
A+IAL &LOW REACTION TURBINE
&f the water parallel to the a0is of the rotation of the shaft, the turbine is known as a0ial flow turbine. #nd if the head at the inlet of the turbine of the turbine is the sum of pressure energy and kinetic energy and during the flow of water through runner a part of pressure energy is converted into kinetic energy, the turbine is known as reaction turbine.
+
For the a0ial flow reaction turbine the shaft of the turbine is vertical. The lower end of the shaft is made larger which is known as KhubN or boss. The vanes are fi0ed on the and hence acts as a runner for a0ial flow reaction turbine. The following are the important type of a0ial flow reaction turbines . Propeller Turbine and +. 4aplan Turbine Pro*eller Tur4!ne'
The vanes are fi0ed to the hub and they are not adustable, the runner is known as propeller turbine.
5a*lan Tur4!ne
The vanes on the hub are adustable the turbine is known as a 4aplan turbine. This turbine is suitable where a large
++
1ain components of 4aplan turbine Fig shows all main parts of a 4aplan turbine. The water from penstock enters the scroll casing and then moves to the guide vanes. Form the guide vanes, the water turns through B88 and flows a0ially through the runner as shown in fig. The discharge through the runner is obtained as M @here $o (uter $iameter of the runner $ b $iameter of hub 5f 5elocity of flow at inlet SPECI&IC SPEED
+2
&t is defined as the speed of a turbine which is identical in shape, geometrical dimensions, blade angles, gate opening etc., with the actual turbine but of such a si9e that it will develop unit power when working under unit head. &t is denoted by the symbol 's. The specific speed is used in comparing the different types of turbines as every type of turbine has different specific speed. &n 1.4.). units, unit power is taken as one horse power and unit head as one meter. ?ut in ).&. units, unit power is taken as one kilowatt and unit head as one meter. Der!/at!on of t%e $*e"!f!" $*ee6
The overall efficiency !η o " of any turbine is given by, η o
=
)haft Power @ater Power Power developed ρ × g × q × H
888
=
ρ × g × q × H
888 @here H Head under which the turbine is working M $ischarge through turbine P Power developed or shaft power. From e
ρ × g × q × H
888
α × Q × H ! as ηo and ρ are constant"
'ow let
+
$ $iameter of actual turbine, ' )peed of actual turbine, u Tangential velocity of the turbine, ' s
=
)pecific speed of the turbine,
5 #bsolute velocity of water.
The absolute velocity, tangential velocity and head on the turbine are related as, u α V,
where 5 α H
α H
?ut the tangential velocity u is given by u
π DN
;8
α DN ∴
From e
H α DN or $α
N
The discharge through turbine is given by M #rea D 5elocity ?ut
#rea α ? D $ α $+
#nd
5elocity α H M α $+ D H
∴
α
α
H N +
H
+
÷ ×
H
N ÷
×
H α
H 2E + N +
)ubstituting the value of M in e
∴
H 2E + N +
× H α
H 6E + = K + N
H 6E + N +
, @here 4 >onstant of proportionality. +6
&f P ,H , the speed ' )pecific speed ' s . )ubstituting these values in the above e
K ×6E + N
P N
+ s
∴
N s
∴
or N s+ = K
+ s
=
H 6E +
or N = + s
N +
N + p H 6E +
=
N
N+ p H 6E +
p
H 6E
&n e
)pecific speed plays an important role for selecting the type of the turbine. #lso the performance of a turbine can be predicted by knowing the specific speed of the turbine. The type of turbine for different specific speed is given in Table 7. as:
)pecific speed ).'o.
Types of turbine !1.4.)."
!).&."
+;
.
8 to 26
7.6 to 28
Pelt on wheel with single et
+.
26 to ;8
28 to 6
Pelton wheel with two or more ets
2.
;8 to 288
6 to ++6
Francis turbine
.
288 to 888
++6 to 7;8
4aplan or propeller turbine
8 Perfor)an"e 9 "%ara"ter!$t!" "ur/e for !fferent tur4!ne$
+A
